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Fighting Fire with Physics

Tuesday, March 06, 2018

On average, about 8 million acres of land burns each year from wildfires. Big fires can reduce forests and grasslands to ash and can destroy homes and lives. Sadly, up to 90 percent of wildland fires in the United States are caused by humans’ carelessness, like unattended campfires, burning trash or waste, tossed-out cigarettes, and arson. The remaining 10 percent are usually started by lightning. Controlling and fighting fires isn’t easy. But knowing the science behind a burning blaze helps firefighters tackle the heat and flames to help save property, land and lives.

Did you know wildfires often want to move uphill? It’s all part of the physics of how fires start and spread.

“The physics of combustion determine when and where we have a fire. Basically, in order to have combustion you need fuel, something burnable; you need oxygen, which we're surrounded by; and then you need a source of energy to kick start the combustion reaction. Now, ‘the how’ fires spread then is also a matter of the physics of fire,” said Don Falk from The University of Arizona.

In general, fire will spread uphill. That’s because fire, like the sun, releases radiant energy that heats up the environment. Some of the energy is dissipated into the sky. But the radiant energy that’s released on the uphill side warms up nearby fuels—like grass and trees.

“That means that those fuels are most likely to combust and the fire's going to creep uphill. So, the fire in effect actually pulls itself uphill by this process of preheating fuels and it's a very powerful force,” said Falk. For every 10 degrees of slope, a fire can double its speed.

“Now, fires can move in other directions too. For example, in Southern California when we have the Santa Ana winds, those are often blowing downhill, and those fires can spread downhill very fast and those are called wind-driven fires, and a wind-driven fire can spread in pretty much any direction,” concluded Falk.

Many scientists think wildfires are likely to become more extreme as global temperatures continue to rise due to greenhouse gas emissions. They emphasize the importance of research to better reduce local risks from fires. Once a fire moves from wildlands into developed areas and neighborhoods, the flames can engulf homes and structures with tragic and costly consequences.